Data for the Science study were gathered by thousands of contact-tracers during India’s lockdown, when mass gatherings were banned, schools were closed and people were ordered to wear face masks in public. Almost 130 million people live in the two Indian states, accounting for about 10 per cent of the country’s population. India has recorded more than 6.2 million Covid-19 cases.

Both states reported their first Sars-CoV-2 infections on March 5. Health workers traced as many as 80 contacts per confirmed case, using skills and resources honed from routinely tracking potential transmitters of HIV and tuberculosis, Laxminarayan said.

Prolific Sars-CoV-2 transmitters tended to spread the virus during prolonged close contact on buses and other forms of transport, according to the researchers, who were also from Princeton University, Johns Hopkins Bloomberg School of Public Health, and Indian state governments. In such settings, there was a 79 per cent chance of an infection occurring.

That compares with only a 1 in 40 chance of catching the virus from someone in the community who wasn’t a household member, Laxminarayan said. Children under 14, though, were found to be frequent “silent” spreaders of the virus, especially to their parents and peers.

“This shows that even without schools being functional, children-to-children transmission seems to be quite important,” he said. “As terrible as it is to say – with two kids at home myself – it’s actually important to keep the kids home.”